(1) Field of the Invention
This invention relates to an apparatus and method for supplying thread in a chain stitch sewing machine that produces over-edging stitches, double chain stitches and other stitches composed of needle and looper threads, in which at least a needle thread is forcibly supplied.
(2) Description of the Related Art
In a chain stitch sewing machine, a needle thread supplied to a needle with too high tension may cause puckering of the sewn fabric, especially a thin fabric. In contrast, a needle thread supplied without tension may cause skip stitches by the loose loops the thread forms.
To avoid such problems, the tension of the needle thread is controlled by a thread tension regulator.
However, the above tension applied to the needle thread greatly varies according to the movement of the needle because the tension is produced by the needle thread, which extends to the needle drop point, being pulled out against the friction with the thread tension regulator. Therefore, the thread tension regulator is not easily adjusted so that good sewing results can be obtained.
On the other hand, proposed in U.S. Pat. No. 5056446 is an automatic thread supply device by which a thread is forcibly supplied, and the supply length of the thread is controlled by electrifying/de-electrifying a solenoid. According to this approach, the thread tension is substantially made zero by the thread being supplied forcibly, and consequently, loosening of the needle thread is avoided by supplying the predetermined length of thread needed to form a stitch. Thus, puckering, skip stitches, and other problems can be decreased.
However, the inventors of this invention have found out it is difficult to obtain stable, good sewing results under different sewing conditions, such as differing thickness of fabrics, by the above approach. After various experiments, the timing of applying the thread tension has proved to greatly affect the sewing appearance.